1. Field of the Invention
This invention relates to estimating ash accumulation and more particularly relates to estimating the ash accumulation in a soot filter.
2. Description of the Related Art
Environmental concerns have motivated the implementation of emission requirements for internal combustion engines throughout much of the world. Governmental agencies, such as the Environmental Protection Agency (“EPA”) in the United States, carefully monitor the emission quality of engines and set acceptable emission standards, to which all engines must comply. Generally, emission requirements vary according to engine type. Emission tests for compression-ignition or diesel engines typically monitor the release of diesel particulate matter (“PM”), nitrogen oxides (“NOx”), hydrocarbons (“HC”), and carbon monoxide (“CO”). Catalytic converters implemented in an exhaust gas after-treatment system have been used to eliminate many of the pollutants present in exhaust gas. However, to remove diesel particulate matter, typically a diesel particulate filter herein referred to as a filter must be installed downstream from a catalytic converter, or in conjunction with a catalytic converter.
A common filter comprises a porous ceramic matrix with parallel passageways through which exhaust gas passes. Particulate matter accumulates on the surface of the filter, creating a buildup that must eventually be removed to prevent obstruction of the exhaust gas flow. Common forms of particulate matter are ash and soot. Ash, typically a residue of burnt engine oil, is substantially incombustible and builds slowly within the filter. Soot, chiefly composed of carbon, can be oxidized and driven off of the filter in an event called regeneration. Various conditions, including, but not limited to, engine operating conditions, mileage, driving style, terrain, etc., affect the rate at which particulate matter accumulates within a diesel particulate filter.
Accumulation of particulate matter typically causes backpressure within the exhaust system that can impair engine performance. Particulate matter, in general, oxidizes in the presence of NO2 at modest temperatures, or in the presence of oxygen at higher temperatures. Excessive soot buildup on the filter can precipitate uncontrolled regeneration of a particulate filter, or, in other words, cause rapid oxidation rates resulting in higher than designed temperatures within the filter. Recovery can be an expensive process.
To prevent potentially hazardous situations, it is desirable to oxidize accumulated particulate matter in a controlled regeneration process before it builds to excessive levels. To oxidize the accumulated particulate matter, temperatures generally must exceed the temperatures typically reached at the filter inlet. Consequently, additional methods to initiate regeneration of a diesel particulate filter must be used. In one method, a reactant, such as diesel fuel, is introduced into an exhaust after-treatment system to generate temperature and initiate oxidation of particulate buildup in the filter. Partial or complete regeneration may occur depending on the duration of time the filter is exposed to elevated temperatures and the amount of particulate matter remaining on the filter. Partial regeneration, caused either by controlled regeneration or uncontrolled regeneration, can contribute to irregular distribution of particulate matter across the substrate of a particulate filter.
Controlled regeneration traditionally has been initiated at set intervals, such as distance traveled or time passed. Interval based regeneration, however, has proven to be ineffective for several reasons. First, regenerating a particulate filter without particulate buildup lessens the fuel economy of the engine and exposes the particulate filter to unnecessary temperature cycles. Secondly, if particulate matter accumulates significantly before the next regeneration, backpressure from blockage of the exhaust flow can negatively affect engine performance. In addition, regeneration (controlled or uncontrolled) of a particulate filter containing large quantities of particulate buildup can potentially cause filter failure or the like. Consequently, particulate filters regenerated on a set interval must be replaced frequently to maintain the integrity of an exhaust gas after-treatment system.
Recently, attempts have been made to estimate the amount of particulate matter accumulated in a particulate filter in order to respond more efficiently to actual particulate buildup. In one method, the backpressure measured across a particulate filter by a differential pressure sensor or algorithm is used to estimate diesel particulate matter accumulation. The soot in the particulate filter may be burned off during regeneration to reduce backpressure in response to a differential pressure-based estimate of the particulate accumulation.
Unfortunately, the accumulation of ash in the particulate filter may distort the differential pressure-based estimate, causing the premature regeneration of the particulate filter. Current methods of estimating accumulated particulates cannot differentiate between soot and ash. In particular, the particulate filter may be regenerated when only a small quantity of soot is present in the filter, expending significant reactant in regeneration for a small improvement in backpressure.
The exhaust gas after-treatment system must also exhibit a maximum allowed backpressure if the engine is to deliver a specified level of power. Unfortunately, the accumulation of ash in the particulate filter increases the backpressure of the exhaust gas after-treatment system. The particulate filter may also require more frequent regeneration to remove soot in order support the maximum allowed backpressure for the engine. An algorithm that distinguishes soot from ash would allow the system to optimize regenerations for soot purposes with regenerations for backpressure purposes, supporting fuel economy, engine performance, and the reliability of aftertreatment systems.
From the foregoing discussion, it should be apparent that a need exists for an apparatus, system, and method that estimate ash accumulation in a particulate filter. Beneficially, such an apparatus, system, and method would enable effective and timely regeneration of a diesel particulate filter based on a more accurate estimate of soot accumulation. In addition, the apparatus, system, and method would increase the fuel economy of a vehicle, extend the life expectancy of a diesel particulate filter, and increase the overall efficiency of an engine.